In recent years, power storage devices such as lithium secondary batteries and lithium-ion capacitors have been developed.
Alkali metal silicate such as lithium silicate can be a precursor in synthesis of lithium transition metal silicate salt, which is a positive electrode active material of power storage devices such as lithium secondary batteries (see Patent Document 1).
For example, lithium transition metal (M) phosphate (LiMPO4) and lithium transition metal (M) silicate (Li2MSiO4), which are alkali metal salts, have been proposed as next-generation positive electrode active materials. Examples of the transition metal M include iron (Fe), manganese (Mn), nickel (Ni), and cobalt (Co).
As compared to an oxide such as LiCoO2, which is a widely used positive electrode active material, LiMPO4 and Li2MSiO4 mentioned above have high thermal stability. Even when an abnormal rise in the temperature of a lithium secondary battery occurs, LiMPO4 and Li2MSiO4 generate less oxygen gas, and therefore an oxidation exothermic reaction of a flammable electrolyte can be suppressed.
In the case of LiMPO4 mentioned above, lithium ions are inserted and extracted by a one-electron reaction, and the theoretical capacity of LiMPO4 is approximately 170 mAh/g. In the case of Li2MSiO4, by contrast, lithium ions are inserted and extracted by a two-electron reaction, and the theoretical capacity of Li2MSiO4 is thus as large as approximately 330 mAh/g. It is known that the use of Li2MSiO4 for a positive electrode active material therefore allows the positive electrode to have high capacity.
Conventionally, a common way of synthesizing Li2MSiO4 utilizes a solid phase reaction. A typical solid phase reaction method includes a step of mixing compounds serving as sources of constituent elements for a long time with the use of a ball mill, and a step of performing heat treatment at 650° C. or higher and heat treatment at a temperature of higher than 1000° C. plural times, where heating time is half a day or longer (see Patent Document 2).
As a method for forming a positive electrode with the use of a positive electrode active material synthesized by a solid phase reaction, a method has been employed in which a synthesized positive electrode active material is ground and used for formation of a positive electrode (see Patent Document 1).